The subject matter disclosed herein relates to steam turbines. Specifically, the subject matter disclosed herein relates to nozzles in steam turbines.
Steam turbines include static nozzle assemblies that direct flow of a working fluid into turbine buckets connected to a rotating rotor. The nozzle construction (including a plurality of nozzles, or “airfoils”) is sometimes referred to as a “diaphragm” or “nozzle assembly stage.” Steam turbine diaphragms include two halves, which are assembled around the rotor, creating horizontal joints between these two halves. Each turbine diaphragm stage is vertically supported by support bars, support lugs or support screws on each side of the diaphragm at the respective horizontal joints. The horizontal joints of the diaphragm also correspond to horizontal joints of the turbine casing, which surrounds the steam turbine diaphragm.
Steam turbine drum nozzles are loaded into the diaphragm (drum) within a circumferential slot or groove. These drum nozzles are assembled similarly to conventional nozzle assemblies, however, these drum nozzles conventionally include a dovetail/hooked interface with the (radially) outer diaphragm ring, and a cover at the opposite end, which defines a radially inner flowpath. These drum nozzle assemblies do not conventionally include an inner diaphragm ring, as the radially inner cover acts to define the flowpath. When loading drum nozzles into the diaphragm ring, the first nozzle proximate one of the horizontal joints is conventionally held in position while a pin is wedged behind the nozzle to hold it in place. The wedge corner of the nozzle dovetail is typically measured and aligned with the horizontal joint of the diaphragm ring. Following placement of the first nozzle, additional nozzles are then placed within the circumferential slot until the half stage (either upper or lower) of the assembly is complete. When the final nozzle is placed in the slot, additional measurements are performed to determine whether and how much that nozzle and/or adjacent nozzles will need to be machined (or replaced with nozzles of a different size) in order to align with the horizontal joint of the diaphragm ring on this other end of the slot. Additionally, nozzle assemblies are designed with a predetermined gap between the upper-half nozzles and the lower-half nozzles proximate the horizontal joint. This gap helps to control the throat passing area, harmonic content and/or twisting of the rings at the horizontal joint. It may be difficult to measure and verify this gap due to the edge on the conventional nozzles, and it may also be difficult to hold the first nozzle in place when additional nozzles are forcibly loaded into the circumferential slot.